eHam

eHam Forums => Antennas and Towers and more => Topic started by: 9A5BDP on February 28, 2013, 03:37:13 PM

I am in process of thinking to put two 20m band W8JK antennas in array. And to be rotatable this means vertical orientation. Distance in between will be aprox lambda/2. Do any of you have some clue how high will be gain of this antenna? Or maybe this antenna exist somewhere.Middle of the dipoles will be on 10m (33ft) high in air.

You will have great problems matching this system effectively, take a look at my qrz page and the phased dipoles depicted.It shows a 40 antenna but the design is valid for any band when lengths are adjusted.I have 3dbd measured gain and achieve 20 to 30 db of front to back consistantly at 45 feet high.The beamwidth is relatively wide and instantly reversable.N4JTE

There are several ways to use W9JKs in an array, as well as several ways to use variations of the W8JK in an array, including W8JKs using half wavelength elements and full wavelength elements. The ARRL Antenna Book has information on them and the expected gains, although they may not have the gains for the heights you are considering. From your description I can't determine which configuration you are considering.

In general, it is difficult to stack horizontal dipoles in a vertical direction and get good gain because the height needed can be a problem. Also when the antennas being stacked are more than one element, the stacking distance becomes even greater. Here is some information on stacking, both broadside and colinear:http://www.w8ji.com/stacking_broadside_collinear.htmThat data is for free space. In an actual implemetation when the bottom antenna gets close to the ground, you can end up with less gain for tha stack than for a single antenna at the top height. About the only way to determine the best configuration is to model it.

For an antenna similar to the one the picture you referenced, a W8JK with half wavelength elements, with a 35 ft boom and the boom at 35 ft height, and the elements separated by 17 ft, the gain should be about 6.4 dBi at 13 degrees elevation, not including feedline and phasing line loss.

That's pretty low gain for a 4 element antenna, but by going to vertical polarization you give up ground gain, which is usually about 5 dB. By comparison a half wave dipole will give about 8 dBi gain but not at that low angle. A half wave dipole at 60 ft would produce about the same gain as this W8JK array at 13 degrees elevation. The dipole would have a wider beamwidth. Vertical polarization does give a low angle of radiation.

The W8JK is basically a bi-directional antenna but it is possible with the correct phasing to make it into a unidirectional antenna with a little more gain, maybe to about 8.2 dBi for your example.

Any of the NEC based programs, such as EZNEC, 4NEC2, or MMANA-GAL can analyze this antenna, but if you haven't used these programs before there is a significant learning curve.

My idea is to build or for start make model of antenna like old mr. Kraus :

http://www.naapo.org/W8JK/Images/JDK0334l.jpg

Please recomend some software for modeling antenna systems to me...

EZNEC and 4nec2 are the two places to start. A limited version of EZNEC comes with ARRL antenna book, and 4nec2 can be downloaded from the author's website (just moved to new website at http://www.qrz.com/4nec2 (http://www.qrz.com/4nec2) ).

Both of these have reasonably user friendly front ends to the underlying NEC2 modeling engine.